European Patent Publication EP 0,105,561 discloses a method for transporting a weft thread through the loom shed of a shuttleless loom by means of a weft thread insertion system having a number of insertion nozzles supplied with a fluid for transporting the weft thread. The speed of the weft thread is being measured as the weft thread travels through the insertion channel. A speed signal representing the weft thread speed is produced and transformed into a control signal for controlling an open loop system for influencing the components of the weft thread insertion system which produce the weft thread insertion speed. First, the speed of each weft thread is measured in the starting phase of the weft insertion cycle. The so received control signal is then processed so that either an auxiliary power source is switched into the weft thread insertion system at an earlier or a later point of time, or that the main power source of the weft thread insertion system is switched off, either earlier or later. The control is such, that the respective weft thread, after it has been released for insertion, passes through the weft thread inserting trajectory within a predetermined time counted from the weft thread release. The just mentioned inserting trajectory is measured between the exit of the main nozzle and the exit of the loom shed.
European Patent Publication EP 0,105,561 also discloses that an auxiliary power source is provided in the form of a second fluid source under pressure, whereby the pressure of the second fluid source is higher than the pressure of the main power source.
It is a disadvantage of the apparatus disclosed in European Patent Publication EP 0,105,561 that the measuring of the speed of the weft thread in the starting phase of the weft thread insertion cycle requires a speed detector that must be positioned within the weaving width near the weft thread insertion system. Such a position has an adverse influence on the appearance of the fabric because it causes an interruption in the fabric appearance. Such an interruption is generally not acceptable.
European Patent Publication EP 0,105,561 is based on the assumption that the insertion time will deviate from a rated insertion time value if the acceleration values of the weft thread also deviate from a rated acceleration value. However, this assumption as presented in the just mentioned European Patent Publication is unfounded because the weft thread flying time is not necessarily dependent on the initial acceleration of the weft thread. Rather, there is a dependency or relationship between the weft thread flying time and the application of the transporting force to the weft thread through the entire length of the weft thread or rather of the weft thread insertion channel.
German Patent Publication (DE-OS) 3,002,862 discloses a weaving loom equipped with an apparatus for inserting the weft thread by means of a medium, whereby the weft thread is transported by a main nozzle and a plurality of auxiliary or relay nozzles until the leading end of the weft thread has reached a catch nozzle. At the moment when the weft thread appears at the catch nozzle, a pick-up or signal generator or transducer produces a signal which is supplied to a comparator. The comparator also receives a signal from a further signal generator or transducer which monitors the loom operational speed. The two signals are compared, whereby the signal representing the loom speed is a rated signal and the result of the comparing is used as an adjustment signal for a pressure controllable valve in a closed loop control. The valve in turn controls the pressure of a medium supplied by a pressure source to the main nozzle and to the auxiliary or relay nozzles.
It is a disadvantage of the apparatus and method disclosed in German Patent Publication (DE-OS) 3,002,862 that the pressure increase in the main nozzle, and thus the increased acceleration of the weft thread in its starting phase, damages the weft thread and can even cause weft thread breaking. An excessive pressure peak in the starting phase of the weft thread insertion has in any case an adverse influence on the running characteristic of the loom and on the quality of the woven fabric.
Another disadvantage of the teachings disclosed in German Patent Publication (DE-OS) 3,002,862 is seen in that a pressure increase in the main nozzle actually lengthens the duration of the blowing which in turn causes the weft thread to flutter at a point of time when the loom shed is being closed. Such a flutter results in a loose weave, because the weft threads are inserted without a sufficient stretching on the insertion side.